1. Field of the Invention
The present invention relates to an acoustic resonator and a manufacturing method therefore, and more particularly to a film bulk acoustic resonator using a piezoelectric material and a manufacturing method therefore.
2. Description of the Related Art
Recently there have been dramatic developments in wireless mobile communication technologies. Such mobile communication technologies require diverse radio frequency (RF) parts that can efficiently transfer information in a limited frequency bandwidth. In particular, the filter of the RF parts is one of key element in mobile communication technologies. This filter serves to filter innumerable waves in air to allow users to select or transfer desired signals, thereby enabling high-quality communications.
Currently, wireless communication RF filters are typically dielectric filters or surface acoustic wave (SAW) filters. A dielectric filter provides high dielectric permittivity, low insertion loss, stability at high temperatures, and is robust to vibration and shock. However, the dielectric filter cannot be sufficiently reduced in size and cannot be integrated with other integrated circuits (ICs), including the recently developed Monolithic Microwave Integration Circuit (MMIC). In contrast, a SAW filter provides a small size, facilitates processing signals, has a simplified circuit, and can be manufactured in mass production in use of semiconductor processes.
Further, the SAW filter provides a high side rejection in a passband compared to the dielectric filter, allowing it to transmit and receive high-quality information. However, a SAW filter's line width for the InterDigital Transducer (IDT) is limited to about 0.5 μm since the process for creating the SAW filter includes a light exposure using ultraviolet (UV) rays. Accordingly, the SAW filter cannot cover the high frequency bands, e.g., over 5 GHz, and it is still difficult to construct the MMIC structure on a semiconductor substrate as a single chip.
In order to overcome the limits and problems as above, a film bulk acoustic resonance (FBAR) filter has been proposed in which a frequency control circuit can be completely constructed in the form of MMIC with other active devices integrated together on the existing Si or GaAs semiconductor substrate.
The FBAR is a thin film device that is low-priced, small-sized, and can be designed to have a high-Q. Thus, the FBAR filter can be used in wireless communication equipment of various frequency bands, for example, ranging from 900 MHz to 10 GHz and military radar. The FBAR can be made an order of magnitude smaller than a dielectric filter or a lumped constant (LC) filter and has a very low insertion loss compared to the SAW filter. The FBAR can be integrated with the MMIC while providing a filter having a high stability and a high-Q factor.
The FBAR filter includes a piezoelectric dielectric material such as ZnO, AIN, or the any appropriate material having a high acoustic velocity.
The piezoelectric material may be directly deposited onto a Si or GaAs semiconductor substrate, e.g., by RF sputtering. The resonance of the FBAR filter arises from the piezoelectric characteristics of the piezoelectric material used therein. More particularly, the FBAR filter includes a piezoelectric film disposed between two electrodes, and generates bulk acoustic waves to induce resonance.
FIG. 1 to FIG. 3 illustrate conventional FBAR structures. FIG. 1 illustrates a cross-section of a conventional membrane-based (or bulk micro-machining-based) FBAR. This membrane-based FBAR includes a silicon oxide layer (SiO2) deposited on a substrate 11 forming a membrane layer 12 on the reverse side of the substrate 11 through a cavity 16 formed by isotropic etching. A resonator 17 includes a lower electrode layer 13 formed on the membrane layer 12, a piezoelectric layer 14 on the lower electrode layer 13, and an upper electrode layer 15 on the piezoelectric layer 14.
The above membrane-based FBAR provides a low dielectric loss of the substrate 11 and less power loss due to the cavity 16. However, the membrane-based FBAR occupies a large area due to the orientation of the silicon substrate, and is easily damaged due to the low structural stability upon a subsequent packaging process, resulting in low yield. Accordingly, recently, an air gap-type and Bragg reflector-based FBARs been created to reduce the loss due to the membrane and simplify the device manufacturing process.
FIG. 2 illustrates a cross-section of a Bragg reflector-based FBAR.
The Bragg reflector-based (or solidly mounted) FBAR includes an acoustic reflector 28 formed on a substrate 21 and a resonator 29 formed on the acoustic reflector 28. The acoustic reflector 28 typically includes a plurality of dielectric layers 22–24 alternating between low impedance and high impedance materials to insure efficient confinement of the acoustic energy in the resonator 29. The resonator 29 includes a lower electrode layer 25, a piezoelectric layer 26 and an upper electrode layer 27. Thus, such a Bragg reflector-based FBAR includes substances having large acoustic impedance difference therebetween disposed in multiple layers 22–24 on the substrate 21, which induce Bragg reflection to resonate due to acoustic waves between lower and upper electrode layers 25 and 27.
Accordingly, the Bragg reflector-based FBAR has a robust structure, does not cause stress due to the bending of the resonator 29, saves manufacturing time, and can withstand external impact. However, the Bragg reflector-based FBAR the thickness adjustments of the layers 22–24 for the total reflection are not easy and the manufacturing cost increases due to the formation of these reflection layers.
FIG. 3 illustrates a cross-section of an air gap-based (or surface micromachining-based) FBAR structure. The air gap-based FBAR has an air gap 36 formed through a sacrificial layer on a substrate 31 using micromachining technologies and has a membrane layer 32, e.g., a silicon oxide film. A resonator 37 is provided on top of the membrane layer 32.
The resonator 37 includes a lower electrode 33, a piezoelectric substance 34, and an upper electrode 35.
Compared with the membrane-based FBAR of FIG. 1, the air gap-based FBAR reduces processing time, eliminates the possible danger due to a hazardous gas used in this etching, has a lower substrate dielectric loss, and occupies a smaller area. However, the manufacturing yield of the air gap-based FBAR is reduced since the structure is easily damaged due to exposing the structure for a long time while the sacrificial layer is removed.
Further, the manufacturing process of the air gap-based FBAR is complicated.